1. Field of the Invention
The present invention relates to methods for evaluating certain characteristics of printed wiring boards including moisture content, delamination, and surface defects, such as scratches. More particularly, the present invention relates to non-destructive methods used to make these evaluations.
2. Description of Related Art
Multilayer ceramic printed wiring boards (PWB's) are playing an increasing role in defense programs to raise the packing density of electronic circuits. The availability of a non-destructive test for identifying potentially defective boards early in the assembly process will greatly improve product reliability. In addition, such non-destructive testing will also be useful for optimizing process variables in the production of such ceramic PWB's.
Tests have shown that humidity exposure in combination with a direct current (DC) bias can cause short circuit failures in certain multilayer PWB's. Accordingly, it is important to make sure that PWB's are moisture free prior to circuit assembly. One option is to subject all printed wiring boards to a heating or dehydration step prior to circuit assembly. This dehydration step is certainly advisable for PWB's which have been exposed to moisture. However, prior to this invention, there was no simple way to tell when moisture removal was complete. Furthermore, many printed wiring boards may have been stored in conditions free from moisture so that drying is not necessary. In order to eliminate uncertainties about the need for drying of PWB's, it would be desirable to provide a simple, convenient and reliable non-destructive screening process which provides information on PWB moisture content.
Multilayer ceramic PWB's are also susceptible to delamination. Although severe cases of delamination can be identified visually, there are certain situations where delamination may be difficult to detect. Accordingly, there is also a need to provide for screening and detection of possible PWB delamination.
PWB's are typically coated with a glass or plastic coating which is designed to prevent moisture from becoming dispersed within the PWB. During transport and handling prior to circuit assembly, this outer protective coating many times becomes scratched. It is not possible by visual inspection alone to determine whether or not the scratches have compromised the moisture protection provided by the coating. Again, it would be desirable to provide a simple, efficient and reliable non-destructive process which could be used to screen such scratched PWB's in order to determine if the protective coating has been damaged sufficiently to allow moisture to enter into the printed wiring board layers.
Further, it would be desirable to provide some type of non-destructive method for monitoring characteristics of PWB's immediately after fabrication in order to optimize fabrication process parameters.